In multi-circuit applications powered by multiple voltage rails, it is sometimes necessary to guarantee that the power sequencing is maintained between two or more independent circuits. For example, a first circuit powered by a first voltage rail may be communicatively coupled to a second circuit powered by a second voltage rail. It may be necessary that the first voltage rail properly power the first circuit before the second circuit uses an output of the first circuit as valid input for further processing.
Many times it is not possible to control a sequence of ramping voltage rails powering different circuits because the voltage rails are generated independent of each other. Because of this condition, it is possible that the second circuit in the above example is powered in advance of the first circuit. In such an instance, if the second circuit uses the output of the improperly powered first circuit, the second circuit will be processing invalid data.
In order to guarantee power sequencing between a first circuit (such as a driver circuit) and the second circuit (such as a controller circuit), traditional architectures have required use of a dedicated pin on both the controller and driver. As an example, a first circuit can include a dedicated output pin used only for the purpose of communicating whether or not a first voltage rail properly powers the first circuit. The second circuit can include a dedicated input pin, which is used only to receive the signal outputted by the first circuit from the output pin. Accordingly, via communications out the output pin of the first circuit to the input pin of the second circuit, the second circuit can be apprised of whether or not the first circuit is properly powered by a respective voltage rail. Such a configuration is undesirable because it requires use of two dedicated pins (that is, an output pin and an input pin) to support communications between the first circuit and the second circuit.
As an alternative, external monitor circuitry with respect to the first circuit can be configured to monitor the first voltage rail and drive a dedicated input pin of the second circuit to notify the second circuit when the voltage rail powering the first circuitry reaches an appropriate voltage level.